In modern, high speed sheet processing machines such as printers, sorters, collators, reproduction machines, etc., a sheet misfeed or multi-fed sheets can seriously impair the operation of the machine. Numerous devices of the type disclosed in U.S. Pat. Nos. 3,768,803 and 390,277 have been proposed to positively separate the top sheet from a stack of sheets and prevent feeding of more than one sheet at a time through the apparatus. In high speed printing machines, one of the most common methods utilized to separate sheets is sniffer tubes which are moved into contact with the top sheet in the stack, lift the sheet by vacuum from the stack and forward it into take-away rolls. Another common means for separating sheets is to employ a feed roll or feed belt in contact with the top sheet in the stack to separate the sheet therefrom. To prevent multi-feeds, a retard pad such as disclosed in U.S. Pat. No. 3,768,803 or a counter-rotating retard roller driven through suitable clutch means may be utilized to prevent the passage of more than one sheet to the take-away rolls downstream therefrom.
It is obvious that sniffer tubes require a source of vacuum and a fairly complex mechanism to move the tubes into and out of contact with the stack for lifting the sheets therefrom and carrying them to the take-away rolls. When utilizing a stationary retard pad along with a feed belt or feed roller, selection of the retard pad material becomes extremely critical due to the possibility of rapid wear of the retard pad or contamination of the pad by paper fibers or other materials utilized in the manufacture of paper such as clay filler. When utilizing a counter-rotating retard roll, it is necessary to provide a separate drive for the retard roll or drive the roll through suitable chains or belts from other drive means in the apparatus. Further, suitable slip clutch means must also be provided, which means are subject to wear, requiring periodic replacement thereof.
Card feeders of the type utilized for punch cards ordinarily employ a stationary gate which is adjusted to provide a slot for passage of cards therethrough which is greater than the thickness of the card but less than the thickness of two cards to prevent feeding of multiple cards. This system is very effective where the thickness of the cards or the sheets is closely controlled.
It is an object of the present invention to provide a sheet feeder which is adapted to handle a wide range of paper weights without adjustment thereto, which is extremely simple and therefore inexpensive, and which is subjected to minimal wear throughout the life of the machine.